The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuits is known in the art. Such technology is of increasing interest in the production of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Thick film multilayer structures based on copper conductors typically are comprised of at least two patterned layers of copper conductor separated by a dielectric layer. The patterned conductor layers are connected by copper deposited in vias in the dielectric layer. Such structures are formed by multiple deposition and firing of layers of copper and dielectric inks.
Such multilayer circuit structures utilizing copper as the conductor metal have a number of problems. The most common is failure caused by the development of electrical shorts due to interactions between flux materials of the copper conductor ink and the dielectric layer which take place during the multiple firings necessary to fabricate a multilayer integrated circuit. Such materials include copper oxide which forms upon exposure of the ink to air or an oxidizing atmosphere and flux materials such as lead oxide and bismuth oxide. These materials will penetrate a porous dielectric material, particularly if it contains large modifier ions such as lead, barium and bismuth.
In complex, multilayer, copper-based circuits, the problems and potential incompatibilities for a given copper conductor ink will vary somewhat depending on how it is being utilized. For example, the initial or bottom layer of copper ink must be formulated to adhere well to the substrate, e.g. alumina, and to have a coefficient of expansion such that it will not separate from the substrate during repeated firings at elevated temperatures. The top layer of copper conductor ink must be formulated to have excellent solderability, yet be resistant to having flux components leach into porous neighboring dielectric layers. If the dielectric layer is sufficiently porous so that flux components can form an unbroken contact between overlying and underlying copper conductors, an electrical short will be produced when the device is operated. The copper conductor inks used for via fill or "post-up" structures between layers of copper conductor have a third set of requirements.
Via-fill copper conductor inks are conventionally printed onto a substrate having a patterned layer of copper conductor alternatively with dielectric inks to incrementally build a thickness of 2 to 20 mils or more. The copper via-fill conductor overlies and connects to the first layer of copper and is surrounded by dielectric material. The coefficient of expansion of conventional copper inks is usually more than twice that of conventional dielectric materials. The effect of this difference is, of course, significantly more pronounced in a thick structure, such as via-fill, which is surrounded by dielectric than in a thin, patterned, copper conductor layer which is sandwiched between two layers of dielectric material. For this reason, copper via-fills, also termed "post-ups", are subject to microcracking where they join the patterned layers of copper conductor.
The microcracking problem alluded to above can be reduced by increasing the amount of vitreous glass in the via-fill formulations. However, this leads to an increased penetration of flux phases into the dielectric material. The use of such low temperature glasses also increases the absorption of glass into the dielectric material. The loss in volume of the copper via resulting from low temperature glass diffusing into the surrounding dielectric, combined with the fact that the copper is continuously being sintered by repeated firings at elevated temperatures, can produce an even more serious result, i.e. the copper post-up contracts to the point where it will separate from one or both of the patterned copper conductor layers it is to connect.
In accordance with this invention, there has been found a formulation for copper via-fill ink which minimizes both of the aforementioned problems.